The metabolic interdependence of neurons and astroglia forms an impOrtant part of a complex reciprocal relationship between these two cell types. C02 fixation, an anaplerotic process, which, is localized in the astroglial cells, plays a major role in this interdependence. Four and five carbon compounds which are derived from C02 fixation are released from the astroglial cells into the extracellular space. Neurons, which are incapable of replenishing their own supply of these essential citric acid cycle intermediates, have high affinity uptake systems for many of the compounds released from the astroglia. Thus, C02 fixation in astroglial cells may serve to replenish these metabolites in both the astroglial cells and neurons. -We have studied the regulation of this process by Ga++, as well as by depolarizing agents including K+. C02 fixation does not appear to be dependent on extracellular Ca++; the level of intracellular products is increased in the presence of K+ but not by veratridine or glutamate. Studies are underway to identify the 14C- labeled products produced by astroglia following incubation with [14C] bicarbonate and to determine which products are found within the cells and which are released. We will also determine whether 14C-labeled products formed in astroglial cells are incorporated into products unique to neurons. Related studies are also underway to determine the products of [l- 13C]-glucose released from astroglial cells that would be available to neurons.